1. Field of the Invention
The present invention generally relates to image forming and, more particularly, to an image forming apparatus that forms an image based on image signals.
2. Description of the Related Art
Image forming apparatuses have become popular that adopt an electrophotographic type or an ink jet type. A certain level of image quality is required for such image forming apparatuses. One of various elements causing deterioration of the image quality is uneven density in a direction of conveying paper (a direction for sub-scanning paper), which is so-called banding. For example, when the image forming apparatus of the electrophotographic type is used, periodically-uneven rotation (periodical variation of rotation speed) of a photosensitive drum, a driving roller of an intermediate transfer belt, a development roller, or a gear generates the banding in a direction of sub-scanning the image.
More specifically, for example, if the uneven rotation of the photosensitive drum is generated, a position at which laser beams write the image is periodically changed.
Further, if the uneven rotation of the driving roller of the intermediate transfer belt is generated, a position onto which the image is transferred is periodically changed. Furthermore, if the uneven rotation of the development roller is generated, a state where the image is developed is periodically changed. These periodical changes appear as the banding on the image, and thus printing quality is deteriorated.
Having the issues described above as a background, for example, Japanese Patent Application Laid-Open No. 2005-010680 discusses a solution. More specifically, Japanese Patent Application Laid-Open No. 2005-010680 discusses a method for reading printing results with a scanner, measuring a strength of the banding, and then performing a correction at a position where the scanning line is written in the direction of sub-scanning the image, to cancel the printing results when the banding has a certain strength or more.
Japanese Patent Application Laid-Open No. 2005-010680 assumes that a similar kind of banding is always generated at any position of a recording medium where printing is performed. However, the similar kind of banding is not always generated at the same position of the recording medium. This is because, although the banding has a predetermined period, a phase of a density change generated at a leading end of the recording medium is not always constant, therefore the phase can be different every time the printing is performed. Thus, a technique discussed in Japanese Patent Application Laid-Open No. 2005-010680 may not be able to appropriately perform a banding correction.